CLASS GASTEROPODA. 95 
mantle, which contains almost always a shell more or less 
developed. Sometimes they are merely enveloped by the 
folded-back edge of the foot. 
These four orders are hermaphrodites, with reciprocal copu- 
Jation. 
The HETEROPODA 
Have the gills on the back, where they form a transverse range 
of little plumes, and in some are protected, as well as a part of 
the viscera, by a symmetrical shell. What distinguishes them 
best is a foot compressed into a thin and vertical fin, at the 
edge of which is often seen a small cupper, the only vestige 
of the horizontal foot of the rest of the class. 
The PECTINIBRANCHIATA 
Have the sexes separate. Their respiratory organs consist 
almost always of gills, composed of small plates, united in the 
form of combs, and are concealed in a dorsal cavity, widely 
open above the head. 
Pretty nearly all of them have turbinated shells, with the 
mouth sometimes entire, sometimes emarginated, sometimes 
provided with a siphon, and most frequently capable of being 
more or less completely closed, with an operculum attached 
to the foot of the animal behind. 
The SCUTIBRANCHIATA ? 
Have gills analogous to those of the pectinibranchiata ; but the 
sexes are united, so that they fecundate themselves without 
mutual co-operation, like the class of the acephala. Their 
shells are very open, and in several in the form of a shield, not 
turbinated. They never have an operculum. 
The CYCLOBRANCHIATA 
Hermaphrodites, like the scutibranchiata, have a shell of one 
